1. Field of the Invention
The present invention relates to a variable resonator, a tunable bandwidth filter, and an electric circuit device using the same.
2. Description of the Related Art
In the field of high frequency radio communication, necessary signals and unnecessary signals are separated by taking out a signal of particular frequency from a large number of signals. A circuit serving the function is called a filter, and is mounted on many radio communication devices.
In a general filter, center frequency, bandwidth or the like representing the characteristics of the filter are invariant. Even if you wish to adapt radio communication devices using such a filter to the application for various frequencies, it is impossible to operate the filter on frequency characteristics other than previously prepared frequency characteristics that each filter has.
Patent literature 1 given below discloses a filter to solve the problem which has resonators using piezoelectric bodies. A bias voltage is applied to the piezoelectric bodies from outside to change the frequency characteristics (resonance frequency) of the piezoelectric bodies and then the bandwidth of the filter is changed.    Patent literature 1: Japanese Patent Application Laid-Open No. 2004-007352
Although the tunable filter disclosed in the Patent literature 1 has a bandwidth as a ladder type filter, the changing width of the center frequency is as small as about 1% to 2% due to the limitation of the characteristics of the piezoelectric bodies. For this reason, variation of bandwidth is also about the same level, and a significant change of bandwidth is not possible.
Further, a method in which a plurality of filters having different combinations of center frequency and bandwidth are prepared and the filters are switched by a switch or the like corresponding to frequency application is easily considered. However, in this method, filters are necessary by the number of desired combinations of center frequency and bandwidth, and thus a circuit size increases. For this reason, the device increases in size.
On the other hand, miniaturization is not the best design. For example, if the filter is designed in order to obtain a desired performance, a circuit size becomes so small that actual manufacture is difficult in some cases.